The present invention relates to semiconductor processing systems, and more particularly, to semiconductor processing systems having removable cassettes for holding processed and unprocessed workpieces.
In order to decrease contamination and to enhance throughput, semiconductor processing systems often utilize one or more robots to transfer semiconductor wafers, substrates and other workpieces between a number of different vacuum chambers which perform a variety of tasks. An article entitled "Dry Etching Systems: Gearing Up for Larger Wafers", in the October, 1985 issue of Semiconductor International magazine, pages 48-60, describes a four-chamber dry etching system in which a robot housed in a pentagonal-shaped mainframe serves four plasma etching chambers and a loadlock chamber mounted on the robot housing. FIG. 1 of the present application illustrates a typical loadlock chamber 10 having a cassette 12 for holding unprocessed wafers 13 to be unloaded by a robot 14 and transferred to various processing chambers (not shown) attached to a mainframe 16.
The loadlock chamber 10 is a pressure-tight enclosure which is coupled to the periphery of the mainframe 16 by interlocking seals 18 which permit the loadlock chamber to be removed and reattached to the mainframe as needed. The cassette 12 is loaded into the loadlock chamber 10 through a rear door (not shown) which is closed in a pressure-tight seal. The wafers are transferred between the mainframe 16 and the loadlock chamber 10 through a passageway 20 which may be closed by a slit valve 22.
Although only eight wafers are illustrated for purposes of clarity, a typical cassette 12 may be initially loaded with as many as 25 or more unprocessed wafers 13 or other workpieces before the cassette is loaded into the loadlock chamber 10. After the loadlock access door is closed and sealed, the loadlock chamber is then pumped by a pump system (not shown) down to the vacuum level of the mainframe 16 before the slit valve 22 is opened. The robot 14 which is mounted in the mainframe 16 then unloads the wafers from the cassette one at a time, transferring each wafer in turn to the first processing chamber. As best seen in FIG. 2, the robot 14 includes a robot hand or blade 24 which is moved underneath the wafer 13 to be unloaded. The robot 14 then "lifts" the wafer 13 from the wafer supports 26 supporting the wafers 13 in the cassette 12. By "lifting," it is meant that either the robot blade 24 is elevated or the cassette 12 is lowered by a suitable lifting mechanism 30 such that the wafer 13 is lifted off the cassette wafer supports 26. The wafer may then be withdrawn from the cassette 12 through the passageway 20 and transferred to the first processing chamber.
Once a wafer has completed its processing in the first processing chamber, that wafer is transferred to the next processing chamber and the robot 14 unloads another wafer from the cassette 12 and transfers it to the first processing chamber. When a wafer has completed all the processing steps of the wafer processing system, the robot 14 typically returns the processed wafer back to the cassette 12 from which it came. Once all the wafers have been processed and returned to the cassette 12, the cassette in the loadlock chamber is removed and another full cassette of unprocessed wafers is reloaded.
One problem associated with systems having single loadlock chamber and cassette is that the robot will run out of unprocessed wafers to unload before all the wafers are processed and returned to the cassette. Processing chambers will become idle as the last wafer works its way through the system. The time necessary for each wafer to pass through each of the processing chambers and to return to the cassette depends upon the number of processing chambers in the system and the time required for each processing step. Some systems have as many as 10 processing chambers in which each processing step can take 2 minutes or more. Thus, in some systems it can take 20 minutes or longer for the last wafer to return to the cassette, during which time the processing of additional wafers is halted.
Furthermore, once the cassette has been filled with the processed wafers, it is necessary to vent the loadlock chamber, remove the old cassette and insert a cassette of unprocessed wafers into the loadlock and then pump down the loadlock chamber to the vacuum level of the robot chamber. Although the increased vacuum isolation provided by such systems can improve product quality, it has been difficult to achieve commercially acceptable throughput for high vacuum processes, such as, for example, physical vapor processes such as sputtering. The time required to pump down the loadlock chambers to the base level, following loading of a cassette of wafers into the loadlock chamber, can be excessive.
In order to increase throughput, it has been proposed to utilize two loadlock chambers as described in U.S. Pat. No. 5,186,718 which is incorporated in its entirety by reference. In such a two loadlock system, both loadlock chambers are loaded with full cassettes of unprocessed wafers. FIG. 3 of the present application illustrates in time line form several processing cycles for such a two loadlock system 100 (FIG. 4) having a mainframe 116 to which a plurality of processing chamber 140 are coupled. As shown in FIG. 3, the robot 14 starts unloading unprocessed wafers from the first loadlock chamber A ("L.L.A."). Following a certain delay, the first wafer will have completed processing by the system 100. At that time, it is believed that the robot typically loads the first processed wafer back into the cassette of loadlock chamber A, the same cassette from which it was unloaded. This process continues until the robot completes unloading the unprocessed wafers from the cassette of loadlock chamber A. However, because the system has two loadlock chambers, the processing chambers are not idled as the last wafer from loadlock chamber A passes through the system. Instead, once the supply of unprocessed wafers from loadlock chamber A is exhausted, the robot begins to unload unprocessed wafers from loadlock chamber B to start another unloadload cycle (cycle 2) as shown in FIG. 3. As the processing of individual wafers is completed, the robot continues to return processed wafers to the cassette of loadlock chamber A until the loadlock chamber A cassette is filled with processed wafers (completing unload-load cycle 1). At that time, the next processed wafer (the first wafer unloaded from loadlock chamber B) will be returned to the cassette of loadlock B from which it was unloaded.
As the robot unloads unprocessed wafers from the cassette of loadlock chamber B and returns processed wafers to the cassette of loadlock chamber B, the slit valve to loadlock chamber A may be closed to permit the loadlock chamber A to be vented and the cassette removed and replaced with a cassette of unprocessed wafers. Once loadlock chamber A has been pumped back down to the base vacuum level and the robot unloads the last unprocessed wafer from loadlock chamber B, the robot can resume unloading unprocessed wafers from the cassette of loadlock chamber A, initiating a third unload-load cycle (cycle 3). Thus, if the loadlock chamber A with the new cassette of unprocessed wafers can be pumped down before the robot finishes unloading the cassette of loadlock chamber B, processing of wafers can be continued uninterrupted.
When the robot has finished loading processed wafers into the cassette of loadlock chamber B ending unload-load cycle 2, the cassette in loadlock chamber B can be removed and replaced with a full cassette of unprocessed wafers. In the meantime the robot continues to unload unprocessed wafers from and load processed wafers into the cassette of loadlock chamber A. In this manner, wafer processing can be maintained more continuously, significantly increasing throughput over the single loadlock systems.
However, it has now been proposed to process two wafers at a time at a particular station to further increase throughput. Such systems would be similar to the system described above but in general the number of processing chambers at each station would be doubled so that two wafers could be processed simultaneously at that station. In addition, the transfer robot may have two parallel sets of wafer blades to enable two wafers to be transferred simultaneously. To take advantage of the increased throughput potential of such a double wafer system, the number of cassettes in the loadlock chambers could be doubled as well. However, to accommodate two cassettes instead of one cassette, it would be necessary to substantially increase the size of each loadlock chamber to as much as double its present size. Coupling two such double size loadlock chambers to the processing system would occupy valuable space that could otherwise be used for processing chambers.